US 2005/0016788 discloses a variable gear ratio steering device (VGRS), which varies a transfer ratio of steering angle between a steering wheel and steered wheels of a vehicle by the use of an electric motor as an actuator. The motor is electronically controlled by an electronic control unit for VGRS (VGRS-ECU).
This VGRS-ECU is operatively connected to the actuator, which includes, for example, the electric motor for generating torque and a reduction device for transferring reduced rotation. The VGRS-ECU receives a detection signal of a sensor, which detects operation of the actuator, calculates a detection value of operation of the actuator based on the detection signal, and controls the actuator of the VGRS based on the calculated detection value. The operation amount of the actuator is calculated by multiplying the rotation angle of a rotor of the motor by a gear ratio of the reduction device, and corresponds to the operation angle of the actuator.
The detection value of the actuator operation amount is therefore one of parameters to be used to control the VGRS, and hence is required to be highly accurate.
As one exemplary related art, as shown in FIG. 12A, a VGRS-ECU 100 may be configured to have two (first and second) microcomputers 101 and 102 on a substrate. The first and second microcomputers 101 and 102 are configured to calculate detection values of actuator operation amount, respectively, and compare the two calculated detection values thereby to enhance the reliability of the calculated detection values.
Specifically, as shown in FIG. 12B, the first and second microcomputers 101 and 102 have first and second CPUs, first and second input circuits, random access memories (RAMs), read-only memories (ROMs), output circuits and the like, respectively.
As shown in FIG. 13 in more detail, the first and second microcomputers 101 and 102 have first and second detection value calculation programs in the ROMs for calculating first and second detection values of the actuator operation amount, respectively. The first and the second programs are the same.
The first microcomputer 101 receives a detection signal from a sensor through the first input circuit and acquires a first initial value y1 corresponding to the detection signal to calculate a first detection value Y1 by the first CPU based on the first detection value calculation program. In this connection, the first CPU executes S101 and S102 shown in FIG. 14.
The second microcomputer 102 receives the detection signal from the sensor through the second input circuit and acquires a second initial value y2 corresponding to the detection signal to calculate a second detection value Y2 by the second CPU based on the second detection value calculation program. In this connection, the second CPU executes S201 and S202 shown in FIG. 14.
The first and second detection values Y1 and Y2 calculated by the first and second CPUs are stored in RAMs of the first and second microcomputers 101 and 102, respectively. In this connection, the first and second CPUs execute S103 and S203 shown in FIG. 14, respectively. The first microcomputer 101 stores a comparison program in its ROM to compare the first and second detection values Y1 and Y2. The second microcomputer 102 transmits the second detection value Y2 to the first microcomputer 101. The first microcomputer 101 executes the comparison operation by using the first and second detection values Y1 and Y2 based on the comparison program. In this connection, the first CPU executes S104 shown in FIG. 14.
If it is determined that there is no significant difference between the first and second detection values Y1 and Y2 (S104: YES), that is, the first and second values are the same or in a predetermined relation, the control for VGRS is continued by using either one of the detection values Y1 and Y2 at S105 shown in FIG. 14. If it is determined that there is a significant difference between the first and second detection values Y1 and Y2 (S104: NO), the control for VGRS is stopped and a predetermined operation such as a fail-safe operation or an open-loop control is executed irrespective of the detection signal of the sensor at S106 shown in FIG. 14.
As described above, the VGRS-ECU 100 is provided with the first and second microcomputers 101 and 102 to improve the reliability of the detection value of the actuator operation amount.
The VGRS-ECU 100 needs, in addition to the two microcomputers 101 and 102, various electronic components such as MOSFETs, capacitors, resistors and the like. These electronic components are also mounted on a substrate and occupies a large amount of space in the VGRS-ECU 100. Since the number of ECUs in the vehicle recently exceeds 100 units, the VGRS-ECU 100 itself is required to be reduced in size. In an instrument panel of the vehicle, a number of devices such as an air-conditioner, meters, audios and the like are mounted at present, and the number will further increase in the future. Therefore, if the VGRS-ECU 100 is mounted in the instrument panel of the vehicle, it must be further reduced in size.
Further, the first and second microcomputers 101 and 102 need be connected by a communication line CL to transfer the detection values Y1 and Y2 between the first and second microcomputers 101 and 102 for the comparison operation in the first microcomputer 101. The communication line CL is susceptible to external noise. Therefore it is required to reconfigure the VGRS-ECU 100 so that the detection values Y1 and Y2 may be monitored each other without transfer of the calculation result.